In general, a memory cell used in a nonvolatile memory has used the charge amount accumulated at a floating gate electrode as information and read the accumulated charge amount as a change in the threshold value of a transistor.
However, when the element area is decreased to improve the performance by reducing chip costs, the charge amount capable of being accumulated in a memory cell decreases. Therefore, it becomes difficult to obtain a voltage difference capable of being sensed sufficiently from the memory cell. Accordingly, the limitations of device scaling have begun to be seen. Therefore, a three-dimensional cell structure has been considered which can not only reduce the area of elements in a conventional plane but also increase the number of mounted elements per unit area by also stacking cells vertically.
Memory cells have used a transistor structure with three terminals as a basic one. However, use of two-terminal elements, such as variable resistive elements, enables further miniaturization of elements to be expected. Variable resistive elements include metal-oxide resistance change memories (ReRAMs), phase-change memories, and conductive bridge memories. Memory cells use the resistance value state of a variable resistive element sandwiched between two terminals as information.
The resistance change memory is a memory that stores information by the generation or disappearance of a filament path formed in a metal oxide film. The metal oxide film has insulation properties immediately after a film formation. However, when a large current is caused to flow by applying a high voltage to the metal oxide film, a microscopic path in which current flows easily in a local part, that is, a so-called filament path, is formed. Generally, it is thought that the composition of a filament path becomes electrically conductive when the path includes excessive metal and that, when a voltage is further applied to the path, oxygen moves depending on conditions, with the result that the path is stabilized, recovering the insulation properties.
In the phase change memory and conductive bridge memory, when the conductive properties in a conductive path and at the path end change, the resistance value of the entire element changes. Particularly in a nonvolatile memory, it is important to repeat a conductive change stably and maintain the conductive change state for a long time stably. To achieve this, it is necessary to control the size of the conductive path, its position, and others.
To realize this, it is necessary to optimize a film forming process itself by uniformizing the film thickness and composition of films constituting a resistance change memory, control a grain size and a shape in the case of crystallization, and prevent impurities from mixing in.
In addition to the above films, the optimization of a cell structure that makes it easier to control a conductive path is desired.